FIG. 1 shows one form of a variable oscillator. The circuit of FIG. 1 includes a resonant portion 1, which produces an oscillating signal at 2, and an amplifying stage 3, which enhances the signal at 2 by sustaining the resonance of the resonant portion and amplifying it to yield on oscillator output signal at 4. The resonant portion comprises a capacitance 5 and an inductance 6 connected in series. The capacitance is a variable capacitance (“varicap”) diode 7 whose capacitance varies in dependence on the voltage applied at a control input 8. Thus the frequency of the oscillator can be varied by means of that voltage, and the oscillator is a voltage-controlled oscillator.
In many applications of oscillators such as the one shown in FIG. 1 there is a need to arrange the oscillator accurately so that a pre-defined range of frequencies can then be tuned over exactly using the varicap diode 7. For example, in many communications applications the oscillator may be required to operate at one of a number of pre-defined frequencies that correspond to the frequencies of available communication channels. In order for the communications terminal that uses the oscillator to establish communication with another terminal the frequencies used by the two terminals must match each other precisely. In production there is often significant variation between the values of the circuit components between individual oscillators. This is especially significant when the oscillator is built on-chip. (Typical variation in the values of on-chip components are: ±30% for resistors, ±10% for capacitors and ±7% for inductors; the values also being strongly dependant on temperature). Therefore, it is common for the oscillator to be trimmed after production so that the pre-determined channel frequencies can then each be selected by applying a corresponding channel-setting voltage at the control input 8.
One way to perform the trimming operation is by using the varicap diode 7 itself. A trimming offset voltage can be applied to the control input 8 to ensure that when the channel-setting voltages are also applied to the control input 8 the pre-determined channel frequencies are generated accurately. However, this approach requires the varicap diodes to have sufficient throw (range) to be capable of adjusting the resonant frequency not just over the frequency envelope of the available channels but also over an additional range to cope with the need for trimming the circuit. The required total throw is typically around 30%. The effect of this is that, compered to one of smaller throw, the varicap diode is more sensitive to the voltage at the control input 8. As a result, in normal operation it is more difficult to control the varicap diode accurately. Furthermore, the wide pull range implies that much of the oscillation energy of the circuit passes via the varicap diode 7, which typically have much higher losses (i,e. lower Q) than fixed value capacitors. High loss causes poor phase noise, which substantially degrades the performance of radio receivers, which are a common application of variable oscillators. Another problem is that if the varicap diode has a large throw then the variation in voltage at the control input 9 as a result of the oscillation can itself alter the capacitance of the varicap and therefore modulate the frequency of the circuit. In addition, where the oscillator is used with a phase-locked loop (PLL) the wide range of effective capacitance of the varicap 7 means that the loop gain of the PLL is subject to variation. This results in poor settling, which is not compatible with the rapid jumps needed for frequency hopping systems. Although this can be addressed by introducing an adjustment for the loop time constant, this is an expensive operation during manufacture.
Another approach is to use the circuit of FIG. 2. In FIG. 2 like components are numbered as for FIG. 1. In the circuit of FIG. 2 an additional mechanical trimmer 9 is provided in the capacitative portion 5 of the oscillator. This allows a varicap diode of smaller throw to be used. However, the mechanical trimmer is bulky, relatively expensive and requires an inconvenient step of mechanical adjustment during production. As an alternative to a mechanical trimmer the capacitance 9 could be provided by a on-chip (monolithic) capacitor that can be adjusted during production by laser trimming. However, this approach is inconvenient because it can only be done before the monolithic component is packaged, and expensive because the laser trimming step has a low yield and is incompatible with conventional integrated circuit (IC) processes.
In another known arrangement the continuously variable capacitance unit (e.g. represented by varicap 7 in FIG. 1) are arranged in parallel with a finitely variable capacitance arrangement which is capable of capacitance steps just a little smaller than the sweep range of the finitely variable capacitance. With this arrangement a first coarse tuning operation can be carried out after manufacture using the finitely variable capacitance arrangement to bring desired capacitances within the sweep range of the continuously variable capacitance. The setting of the finitely variable arrangement is then fixed. Then during use the continuously variable capacitance can be adjusted to give precisely the desired capacitance. However, the coarseness of the first tuning operation means that the continuously variable capacitance frequently has to operate substantially outside its optimum range.
There is a need for a variable frequency oscillator that can be trimmed more easily and economically, without significant deterioration in performance.